TransportContext for Client side

[olamy]

Thanks for adding TransportContext on server-side, a similar feature is missing for client side as well.
My plan is to use a single McpClientTransport assuming it's thread safe and can be re use for diferent authz context.
The only way to customize http headers (needed for authz or whatever other http headers) is to add some value in _meta such:

  McpSchema.CallToolRequest request = McpSchema.CallToolRequest.builder()
          .name("test_add")
          .arguments(args)
          .meta(Map.of("Authorization", "really complicated password"))
          .build();

  McpSchema.CallToolResult result = mcpSyncClient.callTool(request);

Then having a SyncHttpRequestCustomizer

  public static SyncHttpRequestCustomizer getRequestCustomizer() {
      return (builder, method, endpoint, body) -> {
          try {
              if (body != null) {
                  McpSchema.JSONRPCMessage jsonrpcMessage = McpSchema.deserializeJsonRpcMessage(OBJECT_MAPPER, body);
                  if (jsonrpcMessage instanceof McpSchema.JSONRPCRequest jsonrpcRequest
                          && jsonrpcRequest.params() instanceof Map params) {
                      Optional<Map<String, Object>> meta = Optional.ofNullable((Map) params.get("_meta"));
                      Optional<String> authz = Optional.ofNullable(
                              (String) meta.orElse(Collections.emptyMap()).get(AUTHORIZATION_HEADER));
                      authz.ifPresent(s -> builder.header(AUTHORIZATION_HEADER, s));
                  }
              }
          } catch (Throwable e) {
              throw new RuntimeException(e);
          }
      };
  }

it works but looks brittle as the headers are part of the body and it's not possible to customize header with webflux such WebClientStreamableHttpTransport

I was thinking of something such

McpSchema.CallToolResult result = mcpSyncClient.callTool(request, transportContext);

transportContext could be very similar to the one used for server part.

and changing the SyncHttpRequestCustomizer accordingly to accept this new arg? but what about WebClientStreamableHttpTransport.
I'm happy to help here if you tell me if you have any preference for implementing this.

[imod]

@olamy long time no see :)

I'm interested in the exact same - the need to pass the enduser JWT token which is used by the MCP Server to call an other service in the name of the user.

unfortunate something like this does NOT work with SSE:

            val timeSupplier: () -> String = { System.currentTimeMillis().toString() }

            val transport =
                HttpClientSseClientTransport
                    .builder(baseUrl)
                    .sseEndpoint(sseEndpoint)
                    .customizeRequest { req ->
                        req.header("CURRENT_TIME", timeSupplier())
                    }.objectMapper(objectMapper)
                    .build()